1. Field of the Invention
This invention relates to the electrolytic preparation of metals from fused baths, and more particularly to the production of praseodymium from praseodymium oxide, in a molten lithium fluoride-praseodymium fluoride electrolyte.
2. Description of the Art
Rare earth metals, once only scientific curiosities, are finding ever-increasing industrial utility. In particular, recently developed rare earth alloy high-strength permanent magnets have greatly increased the demand for certain rare earths having lower atomic numbers, most notably samarium and neodymium. Samarium-cobalt magnets have become particularly important, due to their very high strengths.
The rare earth praseodymium also forms high-strength magnets, when alloyed with cobalt. In addition, praseodymium is useful in magnets as a replacement for some of the samarium in a samarium-cobalt alloy, due to the relatively higher cost of samarium.
Praseodymium metal is frequently prepared by the metallothermic reduction of a praseodymium halide (such as the fluoride), wherein the halide is loaded into a corrosion-resistant container with an active metal, such as calcium metal, then is heated in an inert atmosphere to temperatures above the melting point of praseodymium (about 935.degree. C.), and held at such temperatures until the praseodymium is reduced to the metal. After cooling the reaction mixture to room temperature, the product is separated from active metal halide slag and the container. This procedure has certain disadvantages, including the limited quantity of metal which can be prepared in a batch and the numerous steps which involve personal attention from an operator.
Rare earth metals have been prepared for quite some time by fused salt electrolysis techniques, several such techniques being reviewed by E. Morrice and M. M. Wong, "Fused-Salt Electrowinning and Electrorefining of Rare-Earth and Yttrium Metals," Minerals Science Engineering, Vol. 11, July 1979 (pages 125-135). Early techniques involved electrolysis of rare earth chlorides, using electrolytes of molten sodium and potassium chlorides; some investigators avoided undesired reactions of the metal products by conducting the electrolysis below the metal melting point, thus producing rare earth metal sponges or nodules.
Other investigators have used molten fluoride electrolytes to produce rare earth metal directly from rare earth oxides. Results for praseodymium are reported by E. Morrice and T. A. Henrie, Electrowinning High-Purity Neodymium, Praseodymium, and Didymium Metals from Their Oxides, U.S. Department of the Interior, Bureau of Mines Report of Investigations 6957, May, 1967. Those workers employed a "thermal gradient" electrolysis cell for the production of praseodymium, in which average electrolyte temperature was 1030.degree. C., but metal product was collected in a cooled area, at an average temperature of 800.degree. C., slightly above the solidus point of the electrolyte. By not maintaining product metal at the high formation temperature, better yields and purity were obtained. The electrolyte used was a mixture of 60 percent by weight PrF.sub.3 and 40 percent by weight LiF.
Further information on electrolyzing praseodymium oxide was reported by E. Morrice, E. S. Shedd, and T. A. Henrie, Direct Electrolysis of Rare-Earth Oxides to Metals and Alloys in Fluoride Melts, U.S. Department of the Interior, Bureau of Mines Report of Investigations 7146, June, 1968. A similar "thermal gradient" technique was described, and this report appears to restate the data for praseodymium production obtained by Morrice and Henrie, supra.
Both of the described praseodymium oxide-to-praseodymium electrolysis reports were confined to small laboratory-scale, batch procedures. To recover product, it was necessary to completely cool the electrolyte, crush it, and separate the product metal nodules. Such procedures generally are not suitable for large-scale, commercial production undertakings, which normally require more continuous, less labor-intensive production methods.
In order to make a more or less continuous process, however, it is necessary to maintain product metal in a molten state, so that the metal can be withdrawn without otherwise affecting the ongoing electrolysis cell operation. Problems observed with such higher-temperature operation involve both higher levels of corrosivity to cell construction materials and product losses, due to competing reactions in the molten electrolyte. One such competing reaction is reaction of praseodymium metal product and/or oxide feed with the electrolyte to form an oxyfluoride-containing sludge. A symptom of this problem is a reduced current efficiency, since produced metal is subsequently being reacted.
Accordingly, it is an object of the present invention to provide an improved electrolytic method for producing praseodymium metal from praseodymium oxide.
An additional object is to provide such an improved method wherein metal is collected and removed from an electrolysis cell, in a molten state.
A further object is to provide such a method wherein the increased cell temperatures, needed to produce molten metal, do not result in uneconomic current efficiency.
These and other important objects of the invention will more clearly appear from consideration of the following disclosure.